1. Field of the Invention
The present invention relates to a hydraulic excavator and, more particularly, to a control apparatus for use on a hydraulic excavator.
2. Description of the Related Art
In general, a hydraulic excavator has a hydraulic pump mounted on a carrier vehicle, for supplying pressurized working fluid to actuators such as a boom actuating hydraulic cylinder or an arm actuating hydraulic cylinder. Spool valves for switching the flowing direction of the working fluid are provided in the fluid passages between the hydraulic pump and the respective actuators. Each spool valve is supplied with a pilot pressure of a level corresponding to the amount of operation of an operation lever for controlling the associated actuator, so that the spool valve is displaced by an amount corresponding to the amount of operation of the operation lever. Consequently, pressurized working fluid is supplied to each actuator at a rate which corresponds to the amount of operation of the associated operation cylinder, whereby the actuator operates at a speed which corresponds to the amount of operation of the operation lever. This general arrangement and operation principle are well known in the art.
The hydraulic pump used on this type of hydraulic excavator is either a constant displacement pump or a variable displacement pump. In the latter case, independent variable displacement pumps are used for the boom actuating cylinder and the arm actuating cylinder. Thus, the displacement rates of the respective hydraulic pumps are controlled to values corresponding to the amounts of operations of the boom operation lever and the arm operation lever.
Another known hydraulic excavator has such a hydraulic system that, while separate hydraulic pumps are arranged for a boom actuating cylinder and an arm actuating cylinder, respectively, one of the hydraulic pumps assists the other as the occasion demands. When a boom operation lever is operated, an associated hydraulic pump (referred to as a "first hydraulic pump", hereinafter) supplies the boom actuating cylinder with the pressurized working fluid through the aforementioned spool valve (this valve is referred to as a "boom spool valve", hereinafter) and, at the same time, working fluid displaced from the hydraulic pump associated with the arm actuating cylinder (this pump will be referred to as a "second hydraulic pump", hereinafter) is caused to join in the working fluid from the first hydraulic pump, so as to be supplied to the boom actuating cylinder together with the fluid from the first hydraulic pump. This hydraulic system realizes a comparatively high speed of boom operation which is conveniently used in, for example, loading of soil on a truck.
When the hydraulic pumps used in the hydraulic system of the type described above, the displacement rates of these pumps are controlled in accordance with the amounts of operations of the associated operation levers. When the fluid joining operation is executed, the second hydraulic pump is required to supply the arm actuating cylinder with the pressurized working fluid in accordance with the operation of the arm operation lever and, at the same time, to the boom actuating cylinder. Thus, the displacement rate of the second hydraulic pump is increased in response to the operation of the boom operation lever. The fluid system for implementing the joining of the working fluid usually employs a boom fluid joining valve and a cut valve. The boom fluid joining valve serves to selectively open and close the fluid passage leading from the second hydraulic pump to the boom actuating cylinder, while the cut valve operates to selectively open and close a fluid passage through which the hydraulic fluid from the second hydraulic pump is returned to and collected in a working fluid tank through the arm spool valve when the latter is in a neutral position as a result of setting of the arm operation lever to a neutral position. By operating the boom fluid joining valve and the cut valve to the opening and closing positions, respectively, it is possible to join the working fluid from the second hydraulic pump in the working fluid from the first hydraulic pump, so that the working fluid from the second hydraulic pump is supplied together with the working fluid from the first hydraulic pump to the boom actuating cylinder.
It is also known to provide, in this type of hydraulic excavator, a returning circuit which allows the hydraulic fluid displaced from the rod-side chamber of the arm actuating cylinder to directly return to the bottom-side chamber of the arm actuating cylinder, for various purposes such as prevention of cavitation which is liable to occur when the arm actuating cylinder is being extended, i.e., when the arm is being pulled, effective use of the hydraulic fluid discharged from the rod-side chamber of the arm cylinder, increase of the operation speed of the arm actuating cylinder, and so forth.
A hydraulic excavator often conducts an operation for horizontally pulling a bucket on the end of an arm, for the purpose of levelling the ground. This operation will be referred to as "horizontal pulling operation". In order that the hydraulic excavator performs such a horizontal pulling operation or action, the boom operation lever and the arm operation lever are operated such that the boom is raised while the arm is pulled. If the amount of operation of the boom operation lever associated with the boom actuating cylinder for raising the boom is equal to the amount of operation of the arm operation lever associated with the arm actuating lever for pulling the arm, the boom actuating cylinder and the arm actuating cylinder are operated such that the bucket is horizontally pulled towards the vehicle. In this case, the pilot pressure supplied to the boom spool valve and the pilot pressure supplied to the arm spool valve are in 1:1 relation to each other.
However, the simultaneous operations of the boom operation lever and the arm operation lever in the boom raising direction and in the arm pulling direction by the same amount, for realizing the horizontal pulling of the bucket, requires a high level of skill and experience. In general, it is difficult to simultaneously operate both the boom operation lever and the arm operation lever in the manner described.
As a consequence, it is often experienced that the amount of operation of one of the boom operation lever and the arm operation lever exceeds the amount of operation of the other, with the result that the speed of operation of the arm actuating cylinder for pulling the arm is too high or too low as opposed to the speed of operation of the boom actuating cylinder for raising the boom.
This problem is serious particularly in the case of a hydraulic excavator of the type in which hydraulic fluid is supplied to the boom actuating cylinder from both the first and second hydraulic pumps during the operation of the boom. In this type of hydraulic excavator, the joining of the working fluid from both hydraulic pumps is carried out over a wide range of operation of the boom operation lever. Consequently, the speed of operation of the boom actuating cylinder for raising the boom tends to be too fast with respect to the arm pulling speed of the arm actuating cylinder, even by a small increment of the operation amount of the boom operation lever.
The same problem is encountered even in the case of a hydraulic excavator of the type which does not employ the function of joining of fluid from both hydraulic pumps. Namely, certain levels of skill and experience is required for simultaneously operating the boom operation lever and the arm operation lever such that the operating speeds of the boom actuating cylinder and the arm Actuating cylinder match with each other to achieve the horizontal pulling action. It is thus often experienced that the operation speed of one of the boom actuating cylinder and the arm actuating cylinder is too fast or too low with respect to the operation speed of the other.
When the operation speed of the arm actuating cylinder is too high with respect to the operation speed of the boom actuating cylinder, the end of the arm tends to be lowered, with the result that the bucket cuts into the ground to roughen the levelled ground surface.
Conversely, a too slow speed of operation of the arm actuating cylinder with respect to the operation speed of the boom actuating cylinder tends to lift the end of the arm away from the ground, with the result that the bucket floats above the ground surface, failing to level the ground. In this state, it is impossible to conduct the levelling.